1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image is transferred to a transfer material.
2. Description of the Related Art
There are known image forming apparatuses including a primary transfer device that electrostatically transfers a toner image carried on a surface of an image bearing member to a belt. As an example of such primary transfer device, there is a known device including a transfer member that forms a transfer electric field while being rubbed against a belt in a secured state as opposed to a device that rotates like a primary transfer roller. Japanese Patent Application Laid-Open No. H11-143249 discloses an invention relating to such type of primary transfer device.
The invention disclosed in Japanese Patent Application Laid-Open No. H11-143249 relates to a primary transfer device including a sheet-like conductive member that is in contact with a belt, a PET film that supports a back surface of the conductive member, and a support member that supports a lower end of the PET film. Upon application of a bias to the conductive member, an electric field for transfer is formed between a photosensitive drum and the conductive member. With such configuration, the electric field makes a toner image on a surface of the photosensitive drum be transferred to a transfer material transported by the belt, thereby an image being formed.
However, in the invention disclosed in Japanese Patent Application Laid-Open No. H11-143249, the conductive member and the PET film have different coefficients of thermal expansion, resulting in the conductive member having an undulating shape in a longitudinal direction (direction parallel to the direction of the axis of the photosensitive drum). This phenomenon will be described more specifically for a case where the coefficient of thermal expansion of the support member is smaller than that of the PET film. When the temperature of the inside of the image forming apparatus main body increases, the support member and the PET film thermally expand. When the PET film and the support member are secured to each other, the PET film is pulled by the support member, thereby suppressing the expansion of the PET film. In a region in which the PET film and the support member are bonded to each other and a region in which the PET film and the support member are not bonded to each other, the PET film has different thermal expansion amounts. Accordingly, the PET film undulates in the longitudinal direction at the boundary between the PET film and the support member.
Because of the bond to the PET film, the conductive member undulates together with the PET film. As described above, the undulating phenomenon of the conductive member occurs due to the differences of the coefficient of thermal expansion among the conductive member, the PET film and the support member. Therefore, where the relationship in magnitude of the coefficients of thermal expansion is opposite to that in the aforementioned case, a similar phenomenon also occurs. When the conductive member undulates, unevenness occurs in the state of contact between the conductive member and the belt in the longitudinal direction (direction parallel to the direction of the axis of the photosensitive drum). Compared to a part in a close contact state, in a part in a loose contact state, the contact resistance between the conductive member and the belt increases. Consequently, unevenness occurs in supply of charge from the conductive member, resulting in uneven images.